Move TestPair & PoolManager to engine (#36797)

* Move TestPair & PoolManager to engine

* Add to global usings

* A

* Move ITestContextLike to engine

* Readd cvars partial class

* cleanup diff

Content.MapRenderer/Program.cs

@@ -9,6 +9,7 @@ using Content.IntegrationTests;
 using Content.MapRenderer.Painters;
 using Content.Server.Maps;
 using Robust.Shared.Prototypes;
+using Robust.UnitTesting.Pool;
 using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.Formats.Webp;